The present invention relates to machines for dropping heavy weights on the ground for carrying out processes of dynamic compaction or dynamic consolidation.
In recent years a process known as dynamic compaction or dynamic consolidation has become widely used for preparing foundation soil prior to construction work taking place. Dynamic compaction or consolidation involves dropping a heavy flat-bottomed weight onto the ground, one or more times from a suitable height. The energy of the impact causes a restructuring of the soil structure and the dropping of the weight is carried out in a predetermined pattern over the ground to be consolidated.
To achieve the correct effect on the ground during dynamic compaction processes the weight must fall freely until it strikes the earth, at which point the rope drum is, of course rotating at a considerably faster speed than, say, when hoisting the weight back up. Once the weight has reached the ground it is necessary to stop further uncoiling of the hoist rope from the hoist drum to avoid slackness and possibly even tangling of the rope and although this can be carried out by the operator using his normal brake, this is most undesirable. Using the operator's normal brake to stop excessive uncoiling of the hoist rope is inaccurate because the operator has to judge the time for operation of the brake extremely accurately, failing which there is either late braking causing excessive uncoiling or, more seriously, early braking which slows the weight prematurely before it reaches the ground. Early braking is also extremely dangerous as the operator's normal brake is usually servo-assisted and if the operator does brake the hoist rope early then the crane may be seriously damaged if the hoist rope stays intact, as could normally be expected to happen, when the weight is stopped during its fall to the ground.